For dad's who want something a bit more advanced that one of those buggy boards to sit at the back of your buggy to carry your older child then perhaps the Swiss Strolli Rider from Happy Stoller might do just the trick.
The Swiss Strolli Rider is a bike seat with a comfortable saddle. With a solid, weather-proof Swiss design, it can quickly hitch onto almost any stroller with a round, square, or oval frame.
The Swiss Strolli Rider also comes with large, durable wheels for quick manoeuvres and its minimal width will not get in the way of pushing your buggy.
It is suitable for children 15 months or older, and up to 44lbs (20kg).